Mars and Europa are givens, but the other two may surprise you.
On February 14, 1990, Voyager 1 took a beautiful snapshot of Earth from a distance of six billion kilometers. The image, which was both arranged and popularized by astronomer Carl Sagan, has become widely known as the "Pale Blue Dot," and its stark minimalism continues to captivate people of all backgrounds, decades after it was taken.
Just within the last week, for example, Fox Searchlight announced a new film named after the photo, and Cornell University inaugurated its new Carl Sagan Institute (formerly Institute for Pale Blue Dots)—a research center devoted to rooting out habitable worlds. It's too soon to tell if the movie will be any good, but there's no question that the news from Cornell is exciting, and bodes well for the ongoing search for life beyond Earth.
Indeed, according to Cornell planetary scientist Jonathan Lunine, there are at least four promising candidates for life within our own solar backyard, just waiting to be explored. Lunine gave a talk outlining this tantalizing quartet at the May 9 launch of the Carl Sagan Institute, and brought me up to speed on which worlds made the cut.
"Mars, Europa, Titan, and Enceladus," he told me over email. "These are places for which there is evidence of past or present habitability, hence sites where life may exist today or existed in the past."
It's no shocker to see Mars top the list, given that people have suspected life might thrive on the Red Planet long before we even began exploring it. This claim is also backed by vast amounts of evidence for liquid water in Mars's past, as well as the Curiosity rover's recent discovery of methane in the air, which could potentially originate from a biological source.
Likewise, Jupiter's ice moon Europa is also a perennial favorite when it comes to alien-hunting, given that it boasts more liquid water than Earth in its subsurface ocean. The Saturnian moon Enceladus may not share Europa's name recognition, but as Lunine explained, it has a more important feature in common with the icy Jovian moon.
"Enceladus [has] a salty sea under its ice crust, organics, and evidence of a hot hydrothermal system—all evidence easily sampled by [the Cassini Saturn orbiter] because the material shoots out of Enceladus forming a plume in space," Lunine told me.
"Cassini has flown through this plume several times and sampled it, yielding many indicators that the subsurface sea could host life," he said. "But Cassini cannot tell us if there is actually life there."
All three of these candidates rely on the assumption that liquid water is an important factor in determining whether a world could host life, but that doesn't rule out the existence of lifeforms on waterless worlds. Indeed, it's just as fascinating to speculate on the nature of water-independent creatures as it is to imagine water-dependent ones, which is what makes Titan, Saturn's largest moon, such an interesting candidate for exploration.
"Titan [has] surface seas of methane and ethane, and an extensive nitrogen atmosphere," said Lunine. "Life in the seas—should it exist—would utilize a different biochemistry than life which arose and evolved in liquid water."
We won't know if any of these four suspects actually do host life without further exploration, but the good news is that scientists are already nailing it on that front. Mars is already home to a bevy of orbiters, landers, and rovers, with more on the way.
NASA is also laying out the groundwork for a Europa mission planned for the 2020s, and though there are unfortunately no plans to visit Titan at the moment, the Cassini-Huygens mission has demonstrated that landing on this curious Saturnian moon is possible.
Meanwhile, Lunine is developing a mission to Enceladus that is explicitly geared towards detecting signs of life. "I am Principal Investigator of a Discovery proposal, Enceladus Life Finder, or ELF, to go back to Saturn and fly through the plume of Enceladus repeatedly using modern advanced compositional instruments to search for life and detail more precisely the nature of the subsurface ocean," he said.
Regardless of whether these efforts succeed in turning up extraterrestrial life on these four worlds, it's exciting that there is such a diversity of promising places to look for it. It also bodes well for the pursuit of identifying life outside the solar system, as well as within it.
The James Webb Space Telescope (JWST), scheduled for launch in 2018, will provide an unprecedented glimpse of exoplanets orbiting alien stars. The telescope will have the potential to detect evidence of life in the atmospheres of these distant worlds, though the odds of finding an Earth analog are low.
"We don't have anything truly Earthlike," said Lunine, who is a part of the JWST team. "But JWST will look at some of the currently known planets and there might always be some surprises."
The search for other so-called "pale blue dots" has clearly matured a great deal in the 25 years since Voyager 1 snapped its famous portrait of Earth. With so many ambitious new missions in the works, including those supported by the newly minted Carl Sagan Institute, it looks as if the decades to come are shaping up to be even more eventful.
As Sagan himself would say: "Somewhere, something incredible is waiting to be known."
Correction: This post has been updated to reflect that fact that the Institute for Pale Blue Dots changed its name to the Carl Sagan Institute.